In which Harry, Hermione, and Ron hold a meeting at
the Hog's Head to discuss with other students the
possibility of Harry giving them Defence lessons,
and Hermione tells Ron and Harry that Ginny is dating
Michael Corner.

Interesting facts and notes about the text of this chapter:

The main event of this chapter is the meeting at the Hog's Head, which
marked the founding of the DA.

...for two whole weeks after her original suggestion.

Consequently, the action of this chapter picks up two weeks after the end
of the preceding chapter.

...Ron had had four more Quidditch practices...

Later in the chapter, it's mentioned that practices are being held twice
a week, so four more Quidditch practices is consistent with about
two weeks having passed.

...all three of them had managed to Vanish their mice in Transfiguration
(Hermione had actually progressed to Vanishing kittens)...

It hasn't yet been established if Vanishing something is permanent. If
this kind of Vanishing is the same as that performed by the Vanishing
Cabinet, it's not.

"Did you know the first weekend in October's a Hogsmeade weekend?"

This would be the first Hogsmeade weekend of the year, which more usually
seems to fall on or about Hallowe'en rather than the first week of October.

...they both had Quidditch practice twice a week....

Wood has been known to hold practice three times a week, but since at least
three members of the team (Angelina and the twins) are supposed to be studying
for NEWTs and two for OWLs (Harry and Ron), most of the Gryffindor players
must have an enormous academic workload this year.

I suppose he was checking for the smell of Dungbombs...

As of OP, we have not yet had a precise description of these magical
gag items; we don't know if they smell before they're used or only
afterward.

...a small inn. A battered wooden sign hung from a rusty bracket over the door,
with a picture on it of a wild boar's head, leaking blood onto the white cloth
around it.

The name of this disreputable inn is very appropriate for a Hogsmeade drinking
establishment: apart from the obvious resonance with Hogsmeade's own name,
a hog's head is a cask of beer.

This introduction to the Hog's Head is a very tidy resolution to
various questions raised by the dragon's egg incident back in PS - this
is where Hagrid won the egg in a card game. Up until now, that
was the only occasion on which this inn had been mentioned by
name. It's a nice touch that Hogsmeade has both a family pub (the
Three Broomsticks) and a dive like this. Note that we still don't
know whether the HQ of the 1612 goblin rebellion
(PA5) was the Hog's
Head, the Three Broomsticks, or yet another establishment. Or
even if that inn was converted into another business, such as
Honeydukes, which might explain why the candy shop has a secret passage
to Hogwarts...

The Hog's Head bar...smelled strongly of something that might have been goats.

There was a man at the bar whose head was wrapped in dirty grey bandages...

As Umbridge reveals later, this was her informant about the meeting: Willy
Widdershins, who used the information to plea-bargain his way out of being
punished for the regurgitating toilet incidents.

...two figures shrouded in hoods sat at a tale in one of the windows...

As Harry observes, he'd have thought they were Dementors if they
hadn't been speaking in strong Yorkshire accents. Although the image
of an off-duty Dementor heading off to the pub and speaking with a
strong accent has a certain appeal...

...a witch with a thick black veil that fell to her toes.

As Sirius reveals later, this is actually Mundungus, incognito
so as to keep an eye on Harry.

I specifically asked Professor Flitwick whether students were
allowed to come to the Hog's Head, and he said yes, but he
advised me strongly to bring our own glasses.

Advice that they appear not to have taken.

The barman...was a grumpy-looking old man with a great deal
of long grey hair and beard. He was tall and thin and looked
vaguely familiar to Harry.

Aberforth appears in the "team photo" of the original
Order of the Phoenix, so Harry has seen his picture before. Also,
the barman's description sounds like a downmarket Dumbledore, with
grey rather than silvery hair and a less cheerful attitude.

...the incoming rush of a crowd of people.

Note the selection of people: the Gryffindor team and their friends,
Ginny's boyfriend and his friends, and the non-Slytherin fifth year prefects and their friends.
(Parvati qualifies on two counts, as Hermione's dormmate and Padma's sister, of course.) I'd
guess that Colin and Dennis are in because Colin and Ginny are classmates.

We know that JKR shifted Corner and his friends from Hufflepuff to
Ravenclaw while writing OP (we know this from interviews showing some
of her working notes). Personally, I suspect that she did this to
try to keep the DA's membership more or less balanced between the three
non-Slytherin Houses.

...Colin and Dennis Creevey...

Since Dennis is only a second-year, having been sorted into Gryffindor the
previous September (GF12), it
seems to be a FLINT that he's in Hogsmeade at all, since in previous years
the school rule was that only third-years and older students could go to
Hogsmeade.

Possibly he had never seen his pub so full.

The kids still have a lot to learn about clandestine activities;
such a large meeting at the Hog's Head is noticeable.

"And did you kill a basilisk with that sword in Dumbledore's office?"
demanded Terry Boot. "That's what one of the portraits on the wall told me
when I was in there last year."

Evidently the portraits don't always sham sleep when visitors are present.
It'd be interesting to know why Terry was in Dumbledore's office; Harry
at this point has only been in there three times - once to be questioned
after the basilisk's attack on Justin and Nick, once to report his dream
from Trelawney's class, and once after the Third Task. It's not usual
for a student to be sent to Dumbledore for disciplinary reasons, for instance.

Justin and Colin both saw the basilisk when it attacked them - through
a ghost and through a camera, respectively - so it seems a bit odd that
they're behaving as though Harry's confrontation with it later is news
to them. Wouldn't they have been told what happened once they were
revived from their Petrification?

A nice touch, that even the wizarding world has mythological
creatures (whereas yeti and the Loch Ness kelpie, for instance,
are quite real).

"They met at the Yule Ball and got together at the end of last year..."

We don't have any particular reason to think that Michael is
in Ginny's year. Since he attended the Yule Ball, in fact,
and was talking to a girl who wasn't his date (Ginny went with
Neville, not Michael), there's a strong possibility that Michael
is in Harry's year or above.

In fact, since he was in Harry's sixth year Potions class, we have
confirmation (in addition to HPM)
that Michael is in Harry's year.

There was an odd feeling in the group now. It was as though they had
just signed some kind of contract.

In fact, of course, they have, thanks to Hermione having bewitched the
parchment.

Note that every Gryffindor in Harry's year, other than Seamus, has
signed up. JKR is setting Seamus up as a red herring for the
candidate of betrayer; after months of meetings when all four of his
dormmates were out until curfew, there's no way he couldn't have known
something was going on, particularly since Dean seems to be his best
mate.

...she paused outside Scrivenshaft's Quill Shop, where there was a handsome
display of pheasant feather quills in the window.

This is the first mention of the use of pheasant feathers as quills.

"Which one was Michael Corner?" Ron demanded furiously.
"The dark one," said Hermione.

Harry, who is horrified at the idea of making a speech
in front of so many people.

Ginny, who breaks up the argument between Hermione and
Luna with a very funny imitation of Umbridge - simultaneously
making everybody laugh and reminding them of what they're
supposed to be doing in the Hog's Head in the first place.

Hermione, who within a month's time goes stammering
Voldemort's name to chastising Ron for flinching
every time she says it.

Ron, who puts his finger on the reason for Sirius' restlessness:
that although he's been on the run for two years, at least he was
free before moving back into Grimmauld Place.

Ron, freaking out at the idea of his little sister having a
boyfriend. It probably doesn't help that he hasn't managed
to get a girlfriend himself yet...